1. Field of the Invention
The present invention relates to image processing for reading images of both sides of a document sheet.
2. Description of the Related Art
As a means for digitally reading an image printed on a document, an image reader mounted on a copying machine, FAX apparatus, scanner, and the like is known. Also, an automatic both-sided reader (automatic duplex reader) which reads images printed on both front and rear faces of a document sheet (to be referred to as “images of both sides” hereinafter) by reversing the front and rear faces of the document sheet by a document reversing unit without the intervention of the user is known.
[Document Reader of Automatic Both-Sided Reader]
FIG. 1 is a schematic sectional view showing the arrangement of a document reader 201.
Pickup rollers 203 pick up document sheets 211 stacked on a document table 202 one by one, and feed them into a reading path. A picked-up document sheet 211 is conveyed in a direction of path 1 formed by rollers 205 shown in FIG. 1 via conveyance rollers 204. The document sheet 211 that has reached a reading position via path 1 is irradiated with light coming from a light source 208 arranged in a reading unit 209 via a platen glass 210, and light reflected by the document sheet 211 enters the reading unit 209.
The reading unit 209 comprises a photoelectric conversion element, and outputs an electrical signal according to the intensity of incoming light. This electrical signal is converted into a digital signal by an analog-to-digital (A/D) converter (not shown), thus obtaining digital image data representing a document image. In this manner, the reading unit 209 reads an image (to be referred to as “front image” hereinafter) printed on the front face of the document sheet 211, which passes through the reading position via path 1. Note that the light source 208 has a spectral distribution nearly equal to the range of visible wavelengths. The intensity of light that enters the reading unit 209 depends on the spectral reflectance distribution of an image printed on the document sheet 211.
When the leading end of the document sheet 211 has reached reverse conveyance/discharge rollers 206, the document sheet is discharged from the document reader 201 up to its trailing end. After that, when the reverse conveyance/discharge rollers 206 rotate in the reverse direction, the document sheet 211 is fed again into the document reader 201, and is guided by a guide 207 into a direction of path 2 formed by the rollers 205.
The document sheet 211 that has passed through path 2 is conveyed by the conveyance rollers 204 to the reading position via path 1 again. Then, the reading unit 209 reads an image (to be referred to as “backside image” hereinafter) printed on the rear face of the document sheet 211. After that, the document sheet 211 is discharged outside the document reader 201 by the reverse conveyance/discharge rollers 206.
The document reader 201 repeats the aforementioned operation to sequentially read images of both sides of a plurality of document sheets stacked on the document table 202.
A merit obtained upon reading images of both sides of a document sheet by the automatic both-sided reader having the document reader 201 shown in FIG. 1 is to be able to automatically read images of both sides without the intervention of the user. Furthermore, since images of both sides are read using the single light source and single reading unit, a reading optical system has a single arrangement, and the geometric and color characteristics of the reading results of the images of both sides match.
On the other hand, since the document sheet is conveyed inside the document reader 201 twice, that is, at the time of reading the front image and at the time of reading the backside image, it takes a lot of time to read images, and the document conveyance arrangement is complicated, resulting in a high probability of occurrence of a paper jam. These are the demerits of the automatic both-sided reader.
[Document Reader of Simultaneous Both-Sided Reader]
A simultaneous both-sided reader (simultaneous duplex reader) that simultaneously reads images of both sides by conveying a document sheet only once is available.
FIG. 2 is a schematic sectional view of a document reader 301 of the simultaneous both-sided reader.
Pickup rollers 203 pick up document sheets 211 stacked on a document table 202 one by one, and feed them into a reading path. A picked-up document sheet 211 is conveyed in a direction of path 3 formed by rollers 205 shown in FIG. 2 via conveyance rollers 204. The document sheet 211 that has reached a first reading position via path 3 is irradiated with light coming from a light source 208 arranged in a reading unit 209 via a platen glass 210, and light reflected by the document sheet 211 enters the reading unit 209. The reading unit 209 reads the front image of the document sheet 211 that passes through the first reading position.
After that, the document sheet 211 reaches a second reading position, and is irradiated with light coming from a light source 303 arranged in a reading unit 304. Light reflected by the document sheet 211 enters the reading unit 304. The reading unit 304 reads the backside image of the document sheet 211 that passes through the second reading position. After that, the document sheet 211 is discharged outside the document reader 301 by discharge rollers 302.
The document reader 301 repeats the aforementioned operation to read images of both sides of a plurality of document sheets stacked on the document table 202 by conveying each document sheet only once.
A merit obtained upon reading images of both sides of a document sheet by the simultaneous both-sided reader having the document reader 301 shown in FIG. 2 is not only to be able to automatically read images of both sides without the intervention of the user. Since this reader reads images of both sides of a document sheet by conveying the document sheet only once, the image reading time can be shortened to improve the performance of the image reader. Also, since only one conveyance path is used, the probability of occurrence of a paper jam can be reduced.
In the following description, a combination of the light source 208 and reading unit 209 will be referred to as reading device A, and a combination of the light source 303 and reading unit 304 will be referred to as reading device B.
Arrangement of Reading Device A
As shown in FIG. 2, reading device A is laid out below the platen glass 210, and when the document sheet 211 is placed on the platen glass 210, reading device A is moved in the sub-scan direction to read an image of one face of the document sheet 211. Note that the reading method of moving the reading device will be referred to as “reading through platen” hereinafter.
Since reading device A used in reading through platen has sufficient margins for its layout space and scan space, it can use either a reduction optical system shown in FIG. 3A or a non-scaled optical system shown in FIG. 3B.
Reading of Image by Reduction Optical System
Image reading of the reduction optical system shown in FIG. 3A is as follows.
The reading unit 209 includes the light source 208 and a reflecting mirror 401, which are mechanically fixed, and the reading unit 209 itself moves in a direction of arrow A shown in FIG. 3A. The reading unit 209, light source 208, and reflecting mirror 401 have a width equal to or larger than the total width of the document sheet 211 to be read. Note that the width corresponds to the depth direction in FIG. 3A.
Light L emitted by the light source 208 illuminates the document sheet 211 via the platen glass 210. The light L reflected by the document sheet 211 is reflected by the reflecting mirror 401, and its optical path is changed. The light L is reduced in scale by a reduction lens (f-θ lens) 403, and enters a photoelectric conversion element 402. Note that the reduction lens 403 reduces, in scale, the light L which has a width equal to or larger than the document sheet 211 in accordance with the width of the photoelectric conversion element 402.
The photoelectric conversion element 402 comprises, for example, a charge coupled device (CCD), and is a semiconductor device which converts incoming light into an electrical signal. Normally, the width of the photoelectric conversion element 402 is narrower than that of the document sheet 211. Therefore, the reduction scale of the reduction lens 402 is decided based on the ratio of the width of the document sheet 211 to that of the photoelectric conversion element 402.
The reading unit 209 and reduction lens 403 of the reduction optical system synchronously move in the direction of arrow A to scan the document sheet 211, thus reading the image on the document sheet 211.
Reading of Image by Non-Scaled Optical System
Image reading of the non-scaled optical system shown in FIG. 3B is as follows.
The reading unit 209 which comprises a contact image sensor (CIS) unit includes the light source 208, a non-scaled lens 204, and the photoelectric conversion element 402, which are mechanically fixed in position, and the reading unit 209 itself moves in a direction of arrow B shown in FIG. 3B. The reading unit 209, light source 208, non-scaled lens 404, and photoelectric conversion element 402 have a width equal to or larger than the total width of the document sheet 211 to be read.
Light L emitted by the light source 208 illuminates the document sheet 211 via the platen glass 210. The light L reflected by the document sheet 211 enters the photoelectric conversion element 402 via the non-scaled lens 404.
In this way, the reading unit 209 as the CIS unit moves in the direction of arrow B to scan the document sheet 211, thus reading the image on the document sheet 211.
Light Source and Photoelectric Conversion Element
In general, upon reading a color image, as the light source 208, a light source that includes the visible wavelength range is used. Also, as the photoelectric conversion element 402, a device which has color separation filters of red (R), green (G), and blue (B) as primary colors, and three arrays of photoelectric conversion element groups corresponding to the filters are used.
Also, a method of using light sources which can respectively emit R, G, and B wavelengths as the light source 208 and a device having a single photoelectric conversion element group as the photoelectric conversion element 402 may be used. That is, in this method, upon scanning the document sheet 211, light sources R, S, and B are turned on in turn, reflected light of each light source is read by the single photoelectric conversion element 402, and R, G, and B signals obtained for the respective light sources are combined to obtain color image data.
Arrangement of Reading Device B
As can be seen from the layout shown in FIG. 2, reading device B hardly adopts a reduction optical system having a long optical distance, and normally adopts a non-scaled optical system.
[Different Reading Characteristics Due to Different Optical Systems]
In this way, when reading device A adopts a reduction optical system and reading device B adopts a non-scaled optical system, the front and backside images have different reading characteristics and particularly different color reproducibilities. Even when both reading devices A and B adopt identical optical systems and identical devices, they may have different reading characteristics and particularly different color reproducibilities due to factors such as the degree of float of a document sheet at the reading positions, the degree of incidence of external light, and the like.
The simultaneous both-sided reader is required to have equal color reproducibilities of images read from the two faces of a document sheet. However, in practice, identical pictures/hues on the two faces of a document sheet may be only backgrounds of a presentation reference, brochure, or the like, and respective pages have different detailed contents even on the presentation reference, brochure, or the like. In other words, it is important to have equal color reproducibilities of images read from the two faces of each document sheet in terms of identical backgrounds over a plurality of pages.
Japanese Patent Laid-Open No. 2005-020224 discloses the following method. That is, characteristic parameters R of reference image data which is prepared in advance are compared with characteristic parameters F of read image data of a front image (to be referred to as “front image data” hereinafter) to correct the characteristic parameters F. After that, the backside image is read, and corrected characteristic parameters F′ of the front image data are compared with characteristic parameters Q of read image data of a backside image (to be referred to as “backside image data” hereinafter) to correct the characteristic parameters Q. As described in this reference, this method reduces the characteristic difference between the front and backside image data.
FIG. 4 shows the sequence of image processing which is assumed to correct the color reproducibility difference in the simultaneous both-sided reader that suffers different color reproducibilities of images of both sides.
A reading unit 501 as reading device A reads the front image of the document sheet 211 to generate Ra, Ga, and Ba signals. Note that the Ra, Ga, and Ba signals are signals which have characteristics depending on reading device A and do not have any significance as a color system specified by Commission Internationale de l'Éclairage (CIE). A color space converter 503 converts the Ra, Ga, and Ba signals into Ra′, Ga′, and Ba′ signals of a color space related to the CIE calorimetric system (e.g., L*a*b*), thus obtaining front image data 505.
A reading unit 502 as reading device B reads the backside image of the identical document sheet 211 to generate Rb, Gb, and Bb signals. These Rb, Gb, and Bb signals also have characteristics depending on reading device B. A color space converter 504 converts the Rb, Gb, and Bb signals into Rb′, Gb′, and Bb′ signals of a color space related to the CIE colorimetric system, thus obtaining backside image data 506.
As the color space converters 503 and 504, a three-dimensional lookup table (3DLUT) which inputs and outputs three signals, or 3×3 or 3×9 matrix arithmetic processing can be applied. Also, since the color space conversion is known to those who are skilled in the art, a detailed description thereof will not be given.
An image which has an 8-bit depth per color of RSB can express about 16 million colors. The 3DLUT is normally configured to have color patches fewer than 16 million patches as samples and to minimize a color difference ΔEave of their averages. Therefore, upon using the 3DLUT or matrix arithmetic processing, it is difficult to attain a color difference ΔE=0 for all colors. That is, the 3DLUT inevitably requires interpolations unless a table having about 16 million entries is prepared, and quantization and interpolation errors corresponding to the table size occur. Of course, the data size of the table having about 16 million entries is impractically as large as 500 MB. On the other hand, when using the matrix arithmetic processing, since RGB and L*a*b* have the nonlinear relationship, it is impossible to attain the color difference ΔE=0.
Since the invention described in Japanese Patent Laid-Open No. 2005-020224 aims at reducing the color reproducibility difference between the images read from the two faces of the document sheet, it is difficult to adjust the color reproducibilities of identical background images read from the two faces of each document sheet over a large number of pages. Also, the reference image data must be prepared in advance, and image correction is done according to the reference image data. Therefore, it is difficult to attain dynamic image correction according to documents.